The development of microporous foams is the subject of substantial commercial interest. Such foams have found utility in various applications, such as thermal, acoustic, electrical, and mechanical (e.g., for cushioning or packaging) insulators, absorbent materials, filters, membranes, floor mats, toys, carriers for inks, dyes, lubricants, and lotions, and the like. References describing such uses and properties of foams include Oertel, G., Polyurethane Handbook, Hanser Publishers, Munich, 1985, and Gibson, L. J.; Ashby, M. F., Cellular Solids. Structure and Properties, Pergamon Press, Oxford, 1988. The term "insulator" refers to any material which reduces the transfer of energy from one location to another. The term "absorbent" refers to materials which imbibe and hold or distribute fluids, usually liquids, an example being a sponge. The term "filter" refers to materials which pass a fluid, either gas or liquid, while retaining impurities within the material by size exclusion. Other uses for foams are generally obvious to one skilled in the art.
Open-celled foams prepared from High Internal Phase Emulsions (hereinafter referred to as "HIPEs") are particularly useful in a variety of applications including absorbent disposable articles (U.S. Pat. Nos. 5,331,015 (DesMarais et al.) issued Jul. 19, 1994, 5,260,345 (DesMarais et al.) issued Nov. 9, 1993, 5,268,224 (DesMarais et al.) issued Dec. 7, 1993, 5,632,737 (Stone et al.) issued May 27, 1997, 5,387,207 (Dyer et al.) issued Feb. 7, 1995, 5,786,395 (Stone et al.) Jul. 28, 1998, 5,795,921 (Dyer et al.) issued Aug. 18, 1998), insulation (thermal, acoustic, mechanical) (U.S. Pat. Nos. 5,770,634 (Dyer et al.) issued Jun. 23, 1998, 5,753,359 (Dyer et al.) issued May 19, 1998, and 5,633,291 (Dyer et al.) issued May 27, 1997), filtration (Bhumgara, Z. Filtration & Separation 1995, March, 245-251; Walsh et al. J. Aerosol Sci. 1996, 27, 5629-5630; published PCT application W/O 97/37745, published on Oct. 16, 1997, in the name of Shell Oil Co.), and various other uses. The cited patents and references above are incorporated herein by reference. The HIPE process provides facile control over the density, cell and pore size and distribution, proportion of cell struts to windows, and porosity in these foams.
An important issue in making HIPE foams commercially attractive is economics. The economics of HIPE foams depend on the amount and cost of the monomers used per unit volume of the foam, as well as the cost of converting the monomers to a usable polymeric foam (process costs). Making, HIPE foams economically attractive can require using: (1) less total monomer per unit volume of foam, (2) less expensive monomers, (3) a less expensive process for converting these monomers to a usable HIPE foam, or (4) combinations of these factors. The monomer formulation and process conditions must be such that the properties of the HIPE foam meet the requirements for the particular application.
The physical properties of the foam are governed by: (1) the properties of the polymer from which the foam is comprised, (2) the density of the foam, (3) the structure of the foam (i.e. the thickness, shape and aspect ratio of the polymer struts, cell size, pore size, pore size distribution, etc.), and (4) the surface properties of the foam (e.g., whether the surface of the foam is hydrophilic or hydrophobic). Once these parameters have been defined and achieved for a particular application, an economically attractive process for preparing the material is desired. A key aspect of this process is the rate of polymerization and crosslinking, together referred to as curing, of the oil phase of a HIPE to form a crosslinked polymer network. Previously, this curing step required that the emulsion be held at an elevated temperature (40.degree. C.-82.degree. C.) for a relatively long period of time (typically from 2 hours to 18 hours or longer). Such long cure times necessitate relatively low throughput rates, as well as high capital and production costs.
Previous efforts to devise commercially successful schemes for producing HIPE foams have involved, for example, pouring the HIPE into a large holding vessel which is then placed in a heated area for curing (see for example U.S. Pat. No. 5,250,576 (Desmarais et al.) issued Oct. 5, 1993). U.S. Pat. Nos. 5,189,070 (Brownscombe et al), issued Feb. 23, 1993; 5,290,820 (Brownscombe et al.) issued Mar. 1, 1994; and 5,252,619 (Brownscombe, et al.) issued Oct. 12, 1993 disclose curing the HIPE in multiple stages. The first stage is conducted at a temperature of less than about 65.degree. C. until the foam reaches a partial state of cure. Then the temperature is increased to between 70.degree. C. and 175.degree. C. to effect final curing rapidly. The whole process takes about 3 hours. Another scheme to produce HIPE foams envisaged placing the emulsion on a layer of impermeable film which would then be coiled and placed in a curing chamber (U.S. Pat. No. 5,670,101 (Nathoo, et al.) issued Sep. 23, 1997). The coiled film/emulsion sandwich could then be cured using the sequential temperature sequence disclosed in the Brownscombe, et al patents discussed above. U.S. Pat. No. 5,849,805 issued in the name of Dyer on Dec. 15, 1998 discloses forming the HIPE at a temperature of 82.degree. C. (pour temperature in Example 2) and curing the HIPE at 82.degree. C. for 2 hours. However, none of these approaches offer the combination of very fast conversion (e.g., in minutes or seconds) from HIPE to polymeric foam that would provide for a relatively simple, low capital process for producing HIPE foams both economically and with the desired set of properties.
The art also discloses using pressure to control the volatility of monomers that, otherwise, would boil off at a suitable polymerization/curing temperature. For example, commonly assigned U.S. Pat. No. 5,767,168, issued to Dyer, et al. on Jun. 16, 1998, discloses the suitability of pressurization to control the volatility of relatively volatile conjugated diene monomers. However, the cure time for the foams disclosed therein is still greater than two hours so there is still substantial opportunity for substantial improvement in curing rate that would improve the economic attractiveness of HIPE foams.
Accordingly, it would be desirable to develop a rapid and efficient process for preparing open-celled polymeric HIPE foam materials with the desired properties.